Thousands of New Yorkers took the streets earlier this month in opposition to a series of tweets by Donald Trump that proposed banning transgender people from military service.
Photograph: Michael N/Pacific/BarcroftImages

Donald Trump’s proposed ban on transgender service members is facing opposition from more than 50 retired generals and admirals who are warning that the discriminatory policy would degrade military readiness and harm morale.

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The top military officials said in a letter published Tuesday that transgender military members “must not be dismissed, deprived of medically necessary health care, or forced to compromise their integrity or hide their identity”. It comes less than a week after the president announced on Twitter that the government would not “accept or allow … transgender individuals to serve in any capacity in the US military”.

The statement – signed by 56 retired admirals and generals and released by the Palm Center, a San Francisco-based research institute – adds to the mounting pressure against Trump to back away from the policy, which has also faced resistance from US defense chiefs, LGBT rights groups and conservative politicians.

“This proposed ban, if implemented, would cause significant disruptions, deprive the military of mission-critical talent and compromise the integrity of transgender troops who would be forced to live a lie, as well as non-transgender peers who would be forced to choose between reporting their comrades or disobeying policy,” the letter said.

Trump’s announcement reversed Barack Obama’s policy in 2016 ending a longstanding ban on trans people serving openly in the military. Although Trump claimed on Twitter that the military “cannot be burdened with the tremendous medical costs and disruption” of trans service members, studies have found that reversing Obama’s action would have negative impacts on the military.

The Tuesday letter cited a 2016 study by the Rand Corporation, which estimated that there are up to 6,630 trans people on active duty and more than 4,000 in the select reserve. That research also estimated that the healthcare for trans people who transition would cost $2.4m-$4m per year, a small fraction of the $6bn the Pentagon spends on medical care for active members of the armed forces.

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“As for ostensible disruptions, transgender troops have been serving honorably and openly for the past year, and have been widely praised by commanders,” the letter said, adding: “Eighteen foreign nations, including the UK and Israel, allow transgender troops to serve, and none has reported any detriment to readiness.”

Gale Pollock, a retired major general who signed the letter, said it would be costly to remove trans service members who were already serving.

“We’ve trained them. We’ve invested money in their talents. We should say thank you and allow them to serve honorably,” she told the Guardian. “We’re proposing taking seasoned people with expertise and just letting them go.

“Policy and military decisions are not done with a tweet,” added Pollock, who served in the army for 36 years.

The letter also referenced commentary from Adm Mike Mullen, former chairman of the joint chiefs of staff, who has called on Congress to oppose Trump’s policy, noting the harms of the rescinded “don’t ask, don’t tell” rule, which barred openly gay military members.

The Pentagon appeared to be caught off guard last week when Trump tweeted about the ban, and Gen Joseph Dunford, the top military officer in the US, said armed forces would continue allowing trans people to serve openly until receiving “direction” from the president on how to implement the new rule.